Broken Alps dual pot for a BSS DPR (hack in images)

[Script]

PROBLEM

Pot shaft broken off
C50K dual, 3mm D-shape, green housing (RK series, I think)
Otherwise unit is fully functional.

Anti-log pots are not that common and dual-ganged C-law pots even more so. Also not so common are 3mm shafts, and if that wasn't enough already, a D-shape with the flat side facing upward is yet another challenge. Anyway, that pot was very likely bulk custom-made by BSS.

I couldn't find a replacement easily, not in old-parts stores here in Japan, not online.
(I probably didn't look hard , closely or long enough, , but time is a factor after all -- and so is cost.)


SOLUTION(s)
- The idea of using a simple extender wouldn't have worked cos simply too long.

- Using another dual C50K might have worked, but leads would have needed to be wired to the PCB. Space behind the faceplate in those BSS units is very tight. And just a different knob...

- Use a dual log pot wired in reverse, eh, not really.

- So, inspired by another thread I drilled a 3mm hole vertically into a 6mm shaft for inserting the broken-off 3mm shaft. Unexpectedly, drilling the hole worked out smoothly but it still didn't solve the problem (for the 'how' and 'why' see below).

- So I finally set out to replace the entire shaft itself of that broken pot to make the otherwise functional dual C50K work again.


TOLD IN IMAGES
...cos it's easier to show than to describe in words
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Drill guide (RC axle extension)


Drilled shaft


Shaft inserted


Fit not

 

[Script]

Hack using another dual pot


Insides of these pots apparently differ, maybe because of different series. The 'new' shaft didn't reach deep enough to actually grab and turn the plastic disk and metal wiper assembly. It needs to extend in a very specific shape, luckily the same as on the wiper assembly from the 'new' shaft. It needed cutting though.


Black assembly adapter inserted


Shaft held in place mechanically. Once at it, I carefully pulled out the shaft and turned the stop position around to get flat D-shape face upward in pot mid position.


Putting it back together with knife and hammer. Cutting into the soft metal and bending it outwards. This makes the shaft hold in place internally again.


Comparison of insides. Looks different, new shaft is longer on inside, but performs identically.

 

[Script]

M1 screws. Bolts destroyed in disassembly.


Untouched dual pot on right.
Fixed C50K dual on left, fully functional.


Nuts on back

 

[Script]

Fit on faceplate assembly, pot not soldered in yet


Hole needed slight widened on faceplate


Widened hole on front


TO DO:
3D printed insert (3mm, D-shape) to be glued into carved out knob.

_______________
Unit just took a big leap up the ladder from 'decommissioned' (cos of one broken pot) to 'maintenance'.

 

[scott2000]

What a save...

 

[k brown]

New series on PBS: "This Old Pot" . . .

 

[Script]

Provided replacement parts are at hand and match, the hack above can be done in one hour.

Yeah, could have reached out to Alps/Alpine Japan and inquire about discontinued or custom pots. But I guess they have minimum quantity for orders. And also, where's the fun in that...

The number of BSS units showing up cos unusable and in need of repair has been slowly rising in Japan over the last couple of years.

 

[Disco Volante]

Great work. Those little pots are fiddly.
I've made similar fixes on older bigger ones. Added a switch on the back, that sort of thing.
It's so entirely satisfying when it works!